© 2021 Elsevier LtdIn gamma-ray spectrometry, the full energy peak efficiency can be determined by experimental, or by Monte Carlo (MC) simulation methods. In experimental methods, reference standard materials including a large number of gamma emitters or mixed gamma emitters in different geometries are requested to determine the efficiency curves. Besides, simulation by MC methods has advantages of modelling any geometries and energy range of interest. However, by simulation it is not easy to determine the efficiency curves due to the lack of information supplied from manufacturers. In this work, it is focused on the effect of dead layer thickness and other geometric parameters of detector crystal on the efficiency of the detector. The general-purpose MC code PHITS and specific-purpose MC code GESPECOR are used to simulate the detector. The experiments are performed by single gamma point sources at a 12.5 cm distance from the end cap of the detector. The results showed that the physical parameters of the detector are different from those provided by the manufacturer. After 15 years of operation of the detector, it is observed that the dead layer thickness increased from 0.7 mm to 1.29 mm. It causes a decrease in the efficiency by a reduction in the active volume of the detector. In addition, it is showed that the round shape of the front edge and bullet shape of the inner hole of the detector crystal have an undeniable effect on efficiency of detector especially for low energy photons. The study shows that the PHITS MC simulation program can be successfully used in obtaining efficiency calibration of HPGe detectors and determining the dead layer thickness.